This invention relates to auxetic materials, in particular, to a material comprising an array of interconnected moisture sensitive auxetic fibers.
Auxetic materials are materials that have a negative or effectively negative Poisson's ratio. In contrast to most conventional materials, auxetic materials possess the property that under a tensile load the material expands perpendicularly to the axis along which the tensile load is applied. In other words, auxetic materials expand as they are stretched. Conversely, materials are also auxetic if a compressive load applied along an axis results in a reduction in the dimension of the material along an axis perpendicular to the axis along which the compressive load is applied. Most materials exhibit a positive Poisson's ratio, this ratio being defined by the ratio of the contractile transverse strain relative to the tensile longitudinal strain.
Prior art auxetic materials are only activated by an applied external force and can essentially be divided into two categories. One category comprises honeycomb like polymeric materials, and the other category comprises materials formed by particles linked by fibrils. However, both of these categories of auxetic materials have significant drawbacks preventing commercialization on an industrial scale. In particular, there are problems in producing such auxetic materials reliably and cost-effectively using techniques which are suitable for commercialization. Recently in order to overcome above problems, a helical fiber was developed. However, the structural characteristics of all prior art auxetic materials made them unsuitable for use in devices or articles that are not exposed to an external force. Thus, what is needed is an auxectic material that not only uses a tensile force to create an auxetic effect and/or pores in the material, but uses a second catalyst for creating an auxetic effect and/or pores in the material.